My pc wont start, it starts to fire up and then stops and a no signal message comes up on the Monitor?

I've checked to see if the cables are loose and this made no difference. Its a pretty old packard bell pc (6 year old) and recently been slowing down, freezing and crashing more and more, but I haven't got the money to pay somebody to sort it out so I would like to try myself although I don't know where to start , any advice would be helpful.